Lithium magnesium ferrite memory core material



United States Patent O 3,370,011 LITHIUM Il/IAGNESEUM FERRITE MEMORY CRE MATERIAL Hiroshi Kitagawa, Kokubunji-machi, Toshio Inoue, Setagaya-ku, Tokyo-to, Susurnu Kurokawa, Kodaira-shi, and Shinkichi Horigome, Kokubunji-machi, Tokyo-t0, Japan, assignors to Kabushiki Kaisha Hitachi Seisalrusho, Tokyo-to, Japan, a joint-stock company of Japan Filed July 29, 1964, Ser. No. 385,987 Claims priority, application .lapan, Aug. 2, 1963, Bti/40,305 8 Claims. (Cl. 252-62.61)

` ABSTRACT OF THE DISCLOSURE A memory core material having rectangular hysteresis of small driving current characteristics which operates in a wide operating temperature range. This material consists of lithium ferrite (LiMFeZt-,OQ as the base, more than l mol percent but less than 10 mol percent of magnesium oxide (MgO), and from trace to 10 mol percent of zinc oxide (ZnO), the total mol percent being 100. These components are mixed, shaped and tired to obtain the memory core material of such desirable characteristics.

This invention relates to memory core materials and more particularly to a new memory core material which is capable of operating over a wide temperature range and, moreover, has the characteristic requiring low driving current.

Core materials of ferrites with rectangular hysteresis loops are widely used as memory elements of computers. Recently, there is a demand for a core material for operation over a wide temperature range (for example, from -60 deg. Cfto +100 deg. C.) without special temperature compensation and, moreover, with still lower driving current, on one hand, for use as a memory core in largecapacity and high-speed computers, particularly computers for controls.

However, with the use of ferrites of the Cu-Mn system or the Mn-Mg system heretofore used extensively as memory element materials, their operational ranges when used as memory devices for computers are ordinarily limited to from zero to 60 deg. C. even with temperature compensation. When these materials are used at a temperature in the neighborhood of 60 deg. C. or a higher temperature, they lose squareness of their characteristic, and, moreover, the variations of their driving currents with temperature are large. Accordingly, these ferrites are not desirable for memory devices of computers.

Basically, when the Curie temperature of a ferrite of a memory core is high, the driving current, in general, is high; that is, these characteristics are mutually conflicting. Accordingly, it is ditiicult to produce a core material having at the same time the combination of a high Curie temperature, lo-w coercive force, squareness, which is an indispensable characteristic of memory cores, and a high iiux density having a relationship to the output voltage. As far as we are aware, it has not been possible, heretofore, to produce a core'material satisfying all of these conditions.

It is a general object of the present invention to overcome the above indicated difliculties.

More specifically, it is an object to provide a memory core material which is capable of operating over a wide temperature range and, moreover, has the characteristic of low driving current, and is capable of substantially satisfying all of the other requirements mentioned above.

It is another object to provide a method for producing a memory core material of the above stated character.

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The foregoing objects and other objects and advantages have been achieved by the present invention, which, briey described, resides in a memory core material produced by adding magnesium oxide and Zinc oxide to lithium ferrite constituting the base and ring the mixture so obtained.

The nature and details of the invention will be more clearly apparent by reference to the following detailed description including examples of typical procedure, when taken in conjunction with the accompanying drawings in which:

FIGURE 1 is a current pulse-time diagram indicating the current pulse sequence for measurement of the characteristics of a core material embodying the invention;

FIGURE 2 is a graphical representation showing memory characteristic curves relating to a core material produced according to Example 1, presented hereinafter, of the invention; and

FIGURE 3 is a similar representation for Example 2, also presented hereinafter, of the invention.

By the present invention, in order to produce a core material operable over a wide temperature range, the characteristic of lithium ferrite wherein it has a Curie temperature of approximately 600 deg. C. and, moreover, has a certain degree of squareness is utilized.

More specically, we have found that, by using this lithium ferrite as a base and adding thereto from 1 to 10 mol percent of magnesium oxide, it is possible to obtain a memory core material which has ample squareness and, moreover, is operable over a wide temperature range. We have further found that, by adding to this lithium-magnesium Iferrite a quantity within the range of from 0 to 10 mol percent of Zinc oxide, it is possible to lower the driving current of the core material. In this case, however, the operational temperature range tends to become slightly narrower. l

As lithium `system ferrites, those of the lithium-nickel system and the lithium-copper system are already known, but these ferrites have never been intended specifically for use as memory core materials for operation over a wide temperature range. In actual fact, these ferrites cannot be said to satisfy the requirement of squareness. Furthermore, there are known ferrites of the lithium-magnesium system. However, the addition of Zinc oxide to a lithiummagnesium system ferrite to lower the driving current, as in the present invention, has never been proposed.

In order to indicate more fully the nature of the present invention, the following examples of practice and results are presented, of which Example l relates to an example of the wide temperature range, memory core material of the invention, and Example 2 relates to the material of Example 1 which is caused to have a low driving current characteristic.

Example 1 Lithium carbonate (LiZCOS) and a-ferric oxide cooled, and ground in a mortar, whereupon a magnetic material having the composition represented by L0.5F2.5O4

was obtained. Various quantities of magnesium oxide (MgO) were added to respective samples of this Utiel-:62.504

3 and mixed therewith to form mixtures of the general formula LiMFeMOd-X( mol percent) MgO These mixtures were respectively formed into granules with a suitable binder, 'and these granules were molded into toroids of such dimensions that the dimensions of each after ring would be 51 mils in outer diameter, 33 mils in inner diameter, and 12 mils in thickness. The toroids so formed were tired at a temperature from 1,100 to 1,200 deg. C. for hours in a current of oxygen gas. Oxygen gas was used because the squareness would be impaired if the toroids of core material were tired 1n air.

The cores so produced were measured for their memory characteristics by using a current pulse sequence as indicated in FIGURE 1. The pulse width was 2.5 microseconds, and the pulse rise time was 0.2 microsecond. Furthermore, measurements were carried out with a disturbance ratio (Id/Im) of 0.5, and disturbed one voltage output dV1 was read out with pulse I, while disturbed zero voltage output dVO was read out with pulse S. Hereinafter, the disturbed one voltage output dV1, the disturbed zero voltage output V0 and the switching time ns at the point where the disturbed zero voltage output dVo increases suddenly will be used for the characteristic values of the core.

The variation in the characteristics when the quantity .X (mol percent) of the above mentioned added magnesium oxide (MgO) is varied is indicated in FIGURE l2. As is apparent from the curves shown therein, the ydisturbed one voltage output dV1 for X :0 is low and is unsuitable, but for X=1.0, dVl suddenly increases and indicates excellent characteristic. This indicates that the squareness has been improved.

When the added quantity of MgO becomes greater than 2.0 mol percent, :ZV1 again decreases, and for values of X greater than 10 mol percent, :IVO increases and becomes unusable.

The atomic ratio between lithium and iron in the composition LiojFezgOI-XMgO of this example is 1:5, but if there is a deviation from this ratio, the characteristics will become poor. The Curie temperature of this Li-Mg system ferrite is higher than 500 deg. C., and the variation With temperature of the disturbed one voltage output dVl and the driving current Im are 0.21%/ deg. C. and 0.17% /deg. C., respectively, which are irnprovements of 1/s to 1/6 of those of conventional core materials. Accordingly, the core material produced according to this example is operable in the temperature range of from -50 to +100 deg. C.

Example 2 The driving current Im of the core of the above Example 1 is, as indicated in FIGURE 1, from 1,100 to 1,200 ma., which is substantially high. In order to decrease this current, zinc oxide (ZnO) was added to the aforementioned Li-Mg system ferrite. The process of producing the cores was the same as that described in Example l. The memory characteristics of the iired core material, of the formula ness, whereby the core becomes unsuitable for use as a memory core.

The addition of ZnO has the further effect of lowering the Curie temperature, which becomes approximately -400 deg. C. for an addition of ZnO of l0 mol percent.

Accordingly, the operational temperature range of the core also becomes narrow. For a core with 10 mol percent of ZnO, the operational temperature range is of the order of from -10 to 60 deg. C.

Furthermore, the rates of variation of the characteristics with temperature also increase with increase in the quantity of ZnO added, becoming for 10 mol percent ZnO addition approximately 0.35 percent/ deg. C., which is approximately twice that for no addition of ZnO. However, even with 10 mol percent of ZnO, the operational temperature range is wide in comparison with those of conventional Mn-Mg cores, and, moreover, with rates of variation with temperature of 1/3, the range of use of the core as a memory core is increased.

Thus, as disclosed above, the present invention provides a memory core material capable of operating over a wire temperature range which heretofore could not be attained irrespective of method or means. The invention further affords a method of lowering as desired the driving current of this core material. Accordingly, the memory core material of the invention fully and practically satisfies the requirements for use in various high-performance computers wherein memory cores are used. Furthermore, the memory core material may be expected to have a wide range of application in devices such as memory devices for development of aeronautical and space sciences.

Since it is obvious that many changes and modifications can be made in the above described details of example procedure without departing from the nature and spirit of the invention, it is to be understood that the invention is not to be limited to the details described herein except as set forth in the appended claims.

We claim:

1. A memory core material for Wide operating temperature range which consists essentially of 1 mol percent to 10 mol percent of magnesium oxide (MgO) and a remainder of lithium ferrite (Li0,5Fe2,5O4).

2. The memory core material for wide operating temperature range as defined in claim 1, wherein the specific mol percent of magnesium oxide is 1.7.

3. A memory core material for wide operating temperature range which consists essentially of 1 mol percent to l0 mol percent of magnesium oxide (MgO), up to 10 mol percent of Zinc oxide (ZnO) and a remainder of lithium ferrite (Li0,5Fe2,5O4).

4. The memory core material for Wide operating ternperature range as dened in claim 3, wherein the specic mol percent of magnesium oxide is 1.7.

5. A method for the production of a memory core material for wide operating temperature range, which comprises: preparing a mixture of mol percent total, consisting of l mol percent to 10 mol percent of magnesium oxide (MgO), and a remainder of lithium ferrite Li05Fe2-5O4); and firing the resulting mixture in a stream of oxygen gas.

6. The method as delined in claim 5, wherein said resulting mixture is rendered into a form of granules through the use of a binder, and the granules are molded into the geometric form of a memory core, and then are tired at a temperature range of 1,100 to 1,200 degrees C. 1n a stream of oxygen gas.

7. A method for the production of a memory core material for wide operating temperature range, which comprises preparing a mixture of 100 mol percent total, consisting of 1 mol percent to 10 mol percent of magnesium oxide (MgO), up to 10 mol percent of zinc oxide (ZnO), and a remainder of lithium ferrite and firing the resulting mixture in a stream of oxygen gas.

8. The method as defined in claim 7, wherein said resulting mixture is Yrendered iinto form of granules 5 through the use of a binder, and the granules are molded into the geometric form of a memory core, and then are red at a temperature range of 1,100 to 1,200 degrees C. in a stream of oxygen gas.

References Cited UNITED STATES PATENTS 6 FOREIGN PATENTS 1,115,324 12/1955 France.

696,216 8/ 1953 Great Britain.

5 HELEN M. MCCARTHY, Primary Examiner.

TOBIAS E. LEVOW, ROBERT D. EDMONDS,

Examiners. 

